Microelectromechanical device provided with an anti-stiction structure, and corresponding anti-stiction method

ABSTRACT

An embodiment of a microelectromechanical device having a first structural element, a second structural element, which is mobile with respect to the first structural element, and an elastic supporting structure, which extends between the first and second structural elements to enable a relative movement between the first and second structural elements. The microelectromechanical device moreover possesses an anti-stiction structure, which includes at least one flexible element, which is fixed only with respect to the first structural element and, in a condition of rest, is set at a first distance from the second structural element. The anti-stiction structure is designed to generate a repulsive force between the first and second structural elements in the case of relative movement by an amount greater than the first distance.

PRIORITY CLAIM

The present application claims the benefit of Italian Patent ApplicationSerial No.: TO2008A000714, filed Sep. 30, 2008, which application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

An embodiment of the present invention relates to amicroelectromechanical device provided with an anti-stiction structureand to the corresponding anti-stiction method.

BACKGROUND

As is known, a microelectromechanical device (MEMS) is constituted byone or more mobile structures provided on a substrate and frequentlyequipped with an actuator and a guide that regulates movement thereof.In general, there are three types of actuators: a first type enablesmovement in a direction parallel to the substrate; a second type enablesmovement in a direction perpendicular to the substrate; whilst a thirdtype enables a rotary movement within a specific range of angles.

A significant defect, which arises in particular conditions in the MEMSdevices considered, is the adhesion (stiction) of the mobile structuresto a fixed element adjacent thereto, or directly to the substrate. It isclear that said phenomenon can lead to serious consequences, even to thepoint of rendering the MEMS systems affected thereby inoperative in anunforeseeable way.

The phenomenon of stiction, in MEMS systems, is generated by the surfaceforces that are exerted between the surfaces of two bodies that are incontact. Of course, the more extensive the area of contact, the greaterthe force of interaction between the surfaces. In addition, furtherfactors that intervene in the phenomenon of stiction, are, among otherthings, the roughness of the surfaces, their degree of wear, the levelof humidity and the temperature of the environment in which themicroelectromechanical structures operate.

Techniques currently used for reduction of the phenomena of stiction inMEMS structures are based upon the reduction of the surfaces of contactand upon low levels of humidity, thus creating conditions that areunfavorable to the occurrence of phenomena of stiction.

However, during use, MEMS structures of a mobile type may come intocontact with further surrounding MEMS structures of a fixed type, forexample, involuntarily on account of shock. Continuous contacts betweenMEMS structures can be the cause of a progressive degradation both ofthe surface of contact of the mobile structures and of the surface ofcontact of the surrounding fixed structures. The formation of particlesof material, that occurs following upon the continuous impacts betweenthe surfaces, is itself a further cause of stiction. Consequently, ithappens that, in these cases, the mobile structures may adhere to thefixed structures, thus jeopardizing their functionality.

SUMMARY

An embodiment of the present invention is a microelectromechanicaldevice and a corresponding method that overcome the drawbacks of theknown art.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of one or more embodiments of the presentinvention, embodiments thereof are now described, purely by way ofnon-limiting example, with reference to the annexed drawings, wherein:

FIGS. 1A-1C show a mechanical model of a MEMS device provided with ananti-stiction structure according to an embodiment of the presentinvention, in three operating steps;

FIG. 2 shows a mechanical model of a MEMS device provided with ananti-stiction structure according to another embodiment of the presentinvention;

FIG. 3 shows a mechanical model of a MEMS device provided with ananti-stiction structure according to a further embodiment of the presentinvention;

FIGS. 4A-4C show simplified top-plan views of a possible implementationof the MEMS device of FIGS. 1A-1C, in the same three operating steps;

FIG. 5 shows a simplified perspective view of a possible implementationof the MEMS device of FIG. 2;

FIGS. 6A-6C show side views of the MEMS device of FIG. 5 duringsuccessive operating steps;

FIG. 7 shows a simplified perspective view of a different implementationof the MEMS device of FIG. 2; and

FIG. 8 shows an overall block diagram of an embodiment of an electronicapparatus incorporating the MEMS device described.

DETAILED DESCRIPTION

FIGS. 1A-1C are schematic illustrations of a mechanical model of anembodiment of an anti-stiction structure 1 for a microelectromechanical(MEMS) device, defined hereinafter simply as device 100, during threeoperating steps.

In detail, the device 100 is represented schematically in its maincomponents and comprises: a first structural element; a secondstructural element; an elastic supporting structure set between thefirst and second structural elements and enabling a relative movementbetween them; and an anti-stiction structure and an arrest structure,which are connected to the first structural element or to the secondstructural element. In particular, the first structural element isformed here by a mobile mass 10, the second structural element is formedhere by a load-bearing structure 6, the elastic supporting structurecomprises one or more suspension springs 12, the anti-stiction structurecomprises at least one flexible element 2, and the arrest structurecomprises a stop element 5.

The suspension springs 12 (just one of which is illustrated) have thefunction of enabling movement of the mobile mass 10 only in pre-setdirections. In the example of FIGS. 1A-1C, the suspension springs 12,which have an elastic constant K_(sm), enable movement of the mobilemass 10 only in a direction u.

The flexible element 2 is anchored to the load-bearing structure 6 andis provided with a resting portion 3. In particular, the flexibleelement 2 is of an elastic type, with an elastic constant K_(f) greaterthan that of the suspension spring 12, for example 10-1000 timesgreater.

The stop element 5 is formed by a rigid structure, for example, by aprojection of the load-bearing structure 6, and has the function oflimiting the movements of the mobile mass 10 and of the correspondingsuspension spring or springs 12 and thus preventing any undesirablefailure. The stop element 5 is anchored to the load-bearing structure 6and provided with a contrast surface 7.

The load-bearing structure 6 may, for example, be a substrate on whichthe anti-stiction structure 1 is provided, an intermediate elementbetween the substrate and the mobile mass 10, or any other structuralelement. Furthermore, the flexible element 2, the stop element 5, andthe mobile mass 10 may be carried by different portions of theload-bearing structure 6.

In conditions of rest, the mobile mass 10 is set at a distance from theflexible element 2 and the stop element 5.

FIG. 1A shows the device 100 in conditions of rest, i.e., in the absenceof external forces F_(u) acting in the direction u on the mobile mass 10(F_(u)=0). In said condition, we shall assume that the mobile mass 10 isset at a first distance I₁ from the flexible element 2 and at a seconddistance I₂ from the stop element 5, with I₁<I₂.

When an external force F_(u)>0 acts in the direction u on the mobilemass 10, the latter undergoes a displacement with consequent reductionof the distances I₁ and I₂. In this step, the flexible element 2 doesnot intervene, and hence does not modify the characteristics ofstiffness and hence of sensitivity of the structure, set in the designstage, by appropriately sizing the elements of the device and inparticular the suspension springs 12.

When the mobile mass 10 displaces by a distance greater than I₁ but lessthan I₂ (FIG. 1B), it initially comes into contact with the restingportion 3 of the flexible element 2, then causes bending of the flexibleelement 2 itself, which generates a braking force that opposes thefurther movement of the mobile mass 10. Since the flexible element 2 iselastic, in the impact the surfaces in contact of the mobile mass 10 andthe flexible element 2 do not degrade, or in any case degrade in aconsiderably reduced way with respect to what would occur in the case ofdirect impact with a rigid element, with a very low constant ofelasticity, for example with the stop element 5.

Even though the external force F_(u) is sufficiently high to bring themobile mass 10 into contact with the stop element 5 (FIG. 1C), onaccount of the braking force, the impact is considerably reduced,consequently reducing the degradation of the mobile mass 10 and of thestop element 5.

It may in any case happen that, following upon an intense use of thedevice 100, the surfaces of contact of the mobile mass 10 and of thestop element 5 wear out, with the consequent formation of a deposit ofparticles of material, and generation of phenomena of stiction. Inpractice, a force of stiction F_(ad) is set up.

However, the flexible element 2 exerts on the mobile mass 10 a repulsiveforce F_(r) of opposite sign with respect to the force of stictionF_(ad). Furthermore, also the suspension spring 12 exerts a force F_(sm)that is of opposite sign to the force of stiction F_(ad).

The total repulsive force F_(rep) is consequently given by the followingformula:

F _(rep) =K _(sm) ·I ₂ +K _(f)·(I ₂ −I ₁).

When the external force F_(u) is removed from the mobile mass 10, theforces acting on the mobile mass 10 are the repulsive force F_(rep) andthe force of stiction F_(ad). By appropriately sizing the device 100, itis possible to cause the repulsive force F_(rep) to be always greaterthan the force of stiction F_(ad) so as to guarantee always separationof the mobile mass 10 from the stop element 5.

FIG. 2 shows a different embodiment of the anti-stiction structure 1.

In this case, the flexible element 2 is set fixed with respect to themobile mass 10, whilst the resting portion 3 has the function of pointof contact with the load-bearing structure 6. In this case, I₁ is thedistance between the resting portion 3 and the load-bearing structure 6,but operation is altogether similar to what has been describedpreviously.

In FIG. 3, the arrest element 5 is formed on the mobile mass 10, and thecontrast surface 7 has the function of point of contact with theload-bearing structure 6. Otherwise, the structure is the same as thatof FIG. 2.

FIGS. 4A-4C show, in top-plan view, a possible implementation of theanti-stiction structure 1 of FIGS. 1A-1C, for example applied to themicroelectromechanical gyroscope described in the patent application No.EP-A-1 1677 073 (U.S. Pat. No. 7,258,008), which are incorporated byreference, in which the load-bearing structure 6 comprises a substrate(just one surface 6 a of which is visible) and a frame 6 b of arectangular shape, and the mobile mass 10 is suspended above the surface6 a via elastic springs 12 carried by the frame 6 b.

In particular, FIGS. 4A-4C show three successive operating conditions ofthe anti-stiction structure 1, corresponding to FIGS. 1A-1C,respectively. According to this embodiment, the flexible element 2 isprovided by a beam element, for example made of monocrystalline orpolycrystalline silicon, having one end anchored to the load-bearingstructure 6 and the resting portion 3 free to move in a plane xy. Thestop element 5 is formed by a projection of the load-bearing structure 6extending towards the mobile mass 10.

The mobile mass 10 is typically set in the same plane xy as the frame 6b and is mobile in the plane xy, ideally in the direction y.

In conditions of rest, when an external force F_(y) acting on the mobilemass 10 is equal to zero (FIG. 4A), the mobile mass 10 is set at adistance from the flexible element 2 and from the stop element 5, and,consequently, the flexible element 2 is at rest.

When an external force F_(y) different from zero acts on the mobile mass2, the suspension spring 12 bends, and the mobile mass 10 comes intocontact with the flexible element 2, but, initially, not with the stopelement 5 (FIG. 4B). If the force is sufficiently high, the mobile mass10 in its movement generates a bending of the flexible element 2 andcomes into contact with the stop element 5, which arrests motion thereof(FIG. 4C). As already explained, the flexible element 2 generates inthis step a repulsive force F_(rep) that opposes the further movement ofthe mobile mass, reducing the speed of impact thereof against the stopelement 5.

As soon as the external force F_(y) terminates, the repulsive forceF_(rep) generated by the flexible element 2 co-operates with the forcegenerated by the suspension spring 12 to bring the mobile mass 10 backinto the state of rest, overcoming the force of stiction F_(ad) and thuspreventing stiction of the mobile mass 10 to the stop element 5.

FIG. 5 shows a further embodiment of an anti-stiction structure 1, whichmay be used with a mobile mass 10 that moves perpendicularly or withrotary movement with respect to the load-bearing structure 6, formedhere by a substrate.

Here, the flexible element 2, with an elongated shape, is fixed to themobile mass 10 and precisely is surrounded by the mobile mass 10 itself,from which it is separated by a trench 23, obtained using micromachiningtechniques of a known type.

In detail, the trench 23 is T-shaped, with a first portion 23 aextending in a transverse direction and from a freely oscillable side ofthe mobile mass 10 and a second portion 23 b extending in a directiontransverse to the first portion 23 a. The flexible element 2 extendsalong the first portion 23 a of the trench 23 and is connected to themobile mass 10 via a second torsional spring 22 extending along thesecond portion 23 b of the trench 23.

The flexible element 2 has a projecting portion formed here by a bump 20extending from a free end 2 a of the flexible element 2, in a directiontransverse to the plane of the mobile mass 10, towards the substrate 6.In practice, the bump 20 can be constituted by a portion of the flexibleelement 2 having a thickness greater than that of the mobile mass 10.

The mobile mass 10 is set at a distance from the substrate 6 and issupported by means of first torsional springs 26 that enable a rotarymovement thereof about an axis of rotation 21.

In a condition of rest (FIG. 6A), when no external force acts on themobile mass 10 (F_(z)=0), the mobile mass 10 is substantially parallelto the substrate 6.

In the presence of a high force F_(z), the mobile mass 10 turns aboutthe axis of rotation 21 until the bump 20 is brought into contact withthe substrate 6 (FIG. 6B).

For sufficiently high external forces F_(z), the flexible element 2bends, generating a braking force on the mobile mass 10, until themobile mass 10 comes into direct contact with the substrate 6 (FIG. 6C).More precisely, just one edge 25 of the mobile mass 10 comes intocontact with the substrate 6; in this case, the surface of the substrate6 facing the edge 25 constitutes the stop element 5.

The impact between the mobile mass 10 and the substrate 6 is reducedthanks to the action of the flexible element 2, which reduces thepossibility of damage and/or wear to the parts that come into contact.

Also in this case, the continuous use of the anti-stiction structure ofFIGS. 4 and 5 can cause wear of the edge 25 of the mobile mass 10 and ofthe substrate 6 that are in contact with one another, favoring theoccurrence of phenomena of stiction. However, also in this case, theflexible element 2 generates a repulsive force F_(rep) that contributesto bringing the mobile mass 10 back into the position illustrated inFIG. 6A.

FIG. 7 shows an alternative embodiment of the flexible element 2 usablewith a mobile mass 10 of the type illustrated in FIGS. 5 and 6A-6C.

According to this embodiment, the flexible element 2 has a projectingportion 2 b extending as a prolongation of the flexible element 2itself, beyond the perimeter of the mobile mass 10, and being henceintegral with the flexible element 2. In practice, the flexible element2 has a total length d such as to enable it to project beyond the edge25. In this way, during rotation about the axis of rotation 21, theprojecting portion 2 b of the flexible element 2 comes into contact withthe substrate 6 before the edge 25, behaving in a way substantiallysimilar to the bump 20 of FIG. 5.

Consequently, the anti-stiction structure 1 described enablesimprovement of the behavior of a generic device 100 in regard to thephenomenon of stiction, limiting the force of contact between two mobilebodies with respect to one another, in particular between a mobile massand a load-bearing element, during use of the device 100 or on accountof undesired accidental shock. In this way, the wear of the surfacesthat come into contact, and the consequent stiction, may be considerablyreduced.

Finally, it is clear that modifications and variations may be made tothe anti-stiction structure 1 described and illustrated herein, withoutthereby departing from the spirit and scope of the present disclosure.

For example, a number of anti-stiction structures 1 may be present for asingle mobile mass 10, set on opposite sides of the mobile mass 10, forexample in a symmetrical way. In this way, since the mobile mass 10 maymove in opposite senses along the same direction (in the plane of theload-bearing structure 6 or perpendicularly thereto), i.e., oscillate inopposite senses, the phenomenon of stiction for either sense of movementmay be reduced.

Likewise, in the example of embodiment of FIG. 5, it is possible toprovide bumps 20 extending from the top side of the flexible element 2so as to reduce stiction of the mobile mass 10 in both directions ofrotation, clockwise and counterclockwise, in the case where this were tobecome necessary.

Furthermore, the load-bearing structure 6 may be any fixed or mobileelement, with respect to which the mobile mass 10 moves and with respectto which it is desired to reduce stiction of the mobile mass 10.

The mobile mass 10 may be provided in the same structural layer of theflexible element 2, as illustrated, or else in a different structurallayer.

The stop element 5 may be provided on the structural element 6 and/or onthe mobile mass 10, and the mobile mass 10 may form part of MEMS devicesof a different type, such as accelerometers, gyroscopes, sensors,micromotors, and the like.

For example, the device 100 may be particularly advantageous for use inan electronic apparatus or system 200 (FIG. 8), of a portable type, forexample a cellphone, a PDA, a palm-top or portable computer, an digitalaudio player, a remote control, a video or photographic camera, etc.,comprising a microelectromechanical device 100 of the type describedpreviously; a biasing circuit 222, designed to supply electrical biasingquantities to the microelectromechanical device 100 (in a way in itselfknown and for this reason not described in detail); an interface circuit224, designed to interface with the microelectromechanical device 100for reading one or more electrical quantities associated therewith (in away in itself known and for this reason not described in detail); and amicroprocessor control unit 225, connected to the interface circuit 224,and designed to superintend general operation of the electronicapparatus 200.

Naturally, in order to satisfy local and specific requirements, a personskilled in the art may apply to the embodiments described above manymodifications and alterations. Particularly, although one or moreembodiments have been described with a certain degree of particularity,it should be understood that various omissions, substitutions, andchanges in the form and details as well as other embodiments arepossible. Moreover, it is expressly intended that specific elementsand/or method steps described in connection with any disclosedembodiment may be incorporated in any other embodiment as a generalmatter of design choice.

1. A microelectromechanical device comprising: a first structuralelement; a second structural element, movable with respect to said firststructural element; an elastic supporting structure extending betweensaid first and second structural elements and configured to enable arelative movement between said first and second structural elements; ananti-stiction structure including at least one flexible element fixed toonly the first structural element and, in a rest condition thereof,arranged at a first distance from the second structural element, saidanti-stiction structure being configured to generate a repulsive forcebetween the first and the second structural elements in the case of arelative movement of an amount greater than said first distance.
 2. Themicroelectromechanical device according to claim 1, wherein the firststructural element comprises a mobile mass, and the second structuralelement comprises a load-bearing structure, or vice versa.
 3. Themicroelectromechanical device according to claim 1, further comprising astop element, of a substantially rigid type, fixed only to one betweensaid first and second structural elements wherein, in a rest conditionof said device, said stop element is arranged at a second distance fromanother structural element between said first and second structuralelements, said second distance being greater than said first distance,and being configured to limit the relative movement between said firstand second structural elements.
 4. The microelectromechanical deviceaccording to claim 3, wherein said stop element is distinct from saidflexible element and directly faces an abutment portion of said anotherstructural element between said first and second structural elements. 5.The microelectromechanical device according to claim 2, wherein saidload-bearing structure comprises a substrate, said movable mass beingsuspended above a surface of said substrate and moving in a directionparallel to said surface of said substrate, wherein said flexibleelement has an elongated shape extending transversely to said directionand is provided with an end forming an abutment surface facing a portionof said first structural element.
 6. The microelectromechanical deviceaccording to claim 2, wherein said load-bearing structure comprises asubstrate, said movable mass being suspended above a surface of saidsubstrate and being mobile in a direction perpendicular to said surfaceof said substrate or being rotatably movable with respect to saidsurface, said flexible element having an elongated shape with a, firstend fixed to said movable mass and a second end provided with aprotruding portion facing and arranged at said first distance from saidsurface.
 7. The microelectromechanical device according to claim 6,wherein said mobile mass has a trench facing a side of the mobile massand said flexible element extends within said trench.
 8. Themicroelectromechanical device according to claim 6, wherein saidprotruding portion comprises a bump extending transversely to saidsurface.
 9. The microelectromechanical device according to claim 7,wherein said protruding portion extends as a prolongation of theflexible element, beyond said side of the mobile mass.
 10. A method foractuation of a microelectromechanical device comprising: a firststructural element; a second structural element, movable with respect tosaid first structural element; an elastic supporting structure,extending between said first and second structural elements andconfigured to enable a relative movement between said first and secondstructural elements; and at least one flexible element, fixed only tothe first structural element; the method comprising: arranging theflexible element in a rest condition thereof at a first distance fromthe second structural element; generating, through the flexible element,a repulsive force between the first and the second structural elementsin case of a relative movement of an amount greater than said firstdistance.
 11. The method according to claim 10, wherein, during saidstep of generating a repulsive force, said flexible element undergoeselastic bending caused by a contact with said second structural element.12. The method according to claim 10, wherein the microelectromechanicaldevice further comprises a stop element, of a substantially rigid type,fixed only to one structural element between said first and secondstructural elements, the method comprising the steps of: arranging, inthe rest condition of said device, said stop element at a seconddistance from the another structural element between said first andsecond structural elements, said second distance being greater than saidfirst distance; limiting the relative movement between said first andsecond structural elements through the stop element when the firststructural element is displaced by an amount equal to said seconddistance.
 13. An apparatus, comprising: a substrate; a mass; a firstelastic member having a first end coupled to the substrate and having asecond end coupled to the mass; and a damping assembly disposed betweenthe mass and the substrate.
 14. The apparatus of claim 13 wherein thesubstrate comprises a semiconductor substrate.
 15. The apparatus ofclaim 13 wherein: the substrate has a surface; the first end of thefirst elastic member is coupled to the surface; and the damping assemblyis disposed between the mass and the surface.
 16. The apparatus of claim13 wherein: the substrate has first and second surfaces; the first endof the first elastic member is coupled to the first surface; and thedamping assembly is disposed between the mass and the second surface.17. The apparatus of claim 13 wherein the first elastic member comprisesa spring.
 18. The apparatus of claim 13 wherein the damping assemblycomprises a second elastic member having a first end coupled to the massand having a second end facing the substrate.
 19. The apparatus of claim18 wherein the second elastic member comprises a spring.
 20. Theapparatus of claim 13 wherein the damping assembly comprises a secondelastic member having a first end coupled to the substrate and having asecond end facing the substrate.
 21. The apparatus of claim 13 whereinthe damping assembly comprises a second elastic member coupled to themass, extending toward the substrate, and spaced from the substrate. 22.The apparatus of claim 13 wherein the damping assembly comprises asecond elastic member coupled to the substrate, extending toward themass, and spaced from the mass.
 23. The apparatus of claim 13, furthercomprising a mass stop disposed between the mass and the substrate. 24.The apparatus of claim 13, further comprising: wherein the substratecomprises a surface that faces the mass; and a mass stop extending fromthe surface of the substrate and spaced from the mass.
 25. The apparatusof claim 13, further comprising: wherein the mass comprises a surfacethat faces the substrate; and a mass stop extending from the surface ofthe mass and spaced from the substrate.
 26. The apparatus of claim 13wherein: the mass includes a plate that is operable to rotate about anaxis of the first elastic member and that has a surface that faces aportion of the substrate; and the damping assembly includes a secondelastic member that is coupled to the plate and that protrudes beyondthe surface.
 27. The apparatus of claim 13 wherein: the mass includes aplate that is operable to rotate about an axis of the first elasticmember and that has an edge that is operable to rotate toward thesubstrate; and the damping assembly includes a second elastic memberthat is coupled to the plate and that protrudes beyond the edge.
 28. Theapparatus of claim 13 wherein the mass includes a plate that issubstantially parallel to a portion of the substrate.
 29. The apparatusof claim 13 wherein: the substrate has first and second substantiallyperpendicular surfaces; the second end of the first elastic member iscoupled to the first surface; and the damping assembly includes a secondelastic member that protrudes from the second surface and is disposedbetween the mass and the first surface.
 30. The apparatus of claim 29wherein the second elastic member comprises a beam.
 31. An integratedcircuit, comprising: a substrate; a mass; a first elastic member havinga first end coupled to the substrate and having a second end coupled tothe mass; and a damping assembly disposed between the mass and thesubstrate.
 32. A system, comprising: a first integrated circuit,comprising a substrate, a mass, a first elastic member having a firstend coupled to the substrate and having a second end coupled to themass, and a damping assembly disposed between the mass and thesubstrate; and a second integrated circuit coupled to the firstintegrated circuit.
 33. The system of claim 32 wherein the first andsecond integrated circuits are disposed on a same die.
 34. The system ofclaim 32 wherein the first and second integrated circuits are disposedon different dies.
 35. The system of claim 32 wherein the secondintegrated circuit comprises a controller.
 36. A method, comprising:applying to a mass in a semiconductor structure a first force that is afirst function of a displacement of the mass from a home position whilethe displacement is within a first range; and applying to the mass asecond force that is a second function of the displacement of the masswhile the displacement is beyond the first range.
 37. The method ofclaim 36 wherein the home position comprises a position in which themass experiences substantially zero net force.
 38. The method of claim36 wherein the second force is greater than the first force.
 39. Themethod of claim 36 wherein: applying the first force comprises applyingthe first force in a direction; and applying the second force comprisesapplying the second force in substantially the direction.
 40. The methodof claim 36 wherein: applying the first force comprises applying thefirst force with a first elastic member; and applying the second forcecomprises applying the second force with the first elastic member and asecond elastic member.
 41. The method of claim 36, further comprising:wherein applying the second force comprises applying the second forcewhile the displacement of the mass is within a second range that isbeyond the first range; and applying a stopping force to the mass whilethe displacement of the mass is beyond the second range.
 42. The methodof claim 36 wherein: the first force is a first function of a rotationaldisplacement of the mass from a home position while the rotationaldisplacement is within a first range; and the second force is a secondfunction of the rotational displacement of the mass while the rotationaldisplacement is beyond the first range.
 43. The method of claim 36wherein: applying the first force comprises pulling the mass with anextended first elastic member; and applying the second force comprisespulling the mass with the extended first elastic member and pushing themass with a compressed second elastic member.
 44. The method of claim 36wherein: applying the first force comprises pushing the mass with acompressed first elastic member; and applying the second force comprisespushing the mass with the compressed first elastic member and with abent second elastic member.
 45. The method of claim 36 wherein: applyingthe first force comprises applying a torque to the mass with a twistedfirst elastic member; and applying the second force comprises applying atorque to the mass with the twisted first elastic member and with a bentsecond elastic member.
 46. The method of claim 36 wherein: applying thefirst force comprises pushing the mass with a compressed first elasticmember; and applying the second force comprises pushing the mass withthe compressed first elastic member and with a compressed second elasticmember.
 47. The method of claim 36 wherein: applying the first forcecomprises pulling the mass with an extended first elastic member; andapplying the second force comprises pulling the mass with the extendedfirst elastic member and with an extended second elastic member.